The 21 year-old has never gone beyond the quarter-finals at a major but claimed a third ATP Masters 1000 title in 2018 and won the ATP Finals to finish the season ranked fourth in the world.
Ferrero believes Zverev will make a deep run at one of the four headline tournaments next year but acknowledged he must become far more consistent across five-set matches.
Asked if Zverev will play a grand slam final in 2019, Ferrero, speaking courtesy of the Champions Tennis at the Royal Albert Hall, told Omnisport: "Yes. No doubt, I think he has got the potential – more than any other young player.
"But so far, he finds it difficult to be focused on the court. Last year he lost games in Roland Garros, he played five-set matches against people he's supposed to beat.
"Then in the US Open he lost against [Philipp] Kohlschreiber, in a [third-round] match that theoretically he was superior. So, in grand slams, mentally it cost him a lot.
"Keeping the same intensity in long matches at grand slams is his pending subject. He is a player that is used to playing an amazing set, then a not very good set, another better, the other bad. So, to keep consistency from the first to the last set, it is something he has to improve."
Ferrero thinks Zverev is the only young player on the ATP Tour capable of winning majors on the scale of iconic trio Roger Federer (20), Rafael Nadal (17) and Novak Djokovic (14).
"Zverev will be 22 next year, but he hasn't won a grand slam yet. He has to hurry up and win titles," said the 2003 French Open winner.
"When he can go a step forward at grand slams, because he still hasn't gone beyond the quarter-finals, it will be faster.
"At the moment, I can't see any other players reaching the numbers of Roger, Rafa or Novak."
Zverev split with Ferrero following what he described as a "fight" after this year's Australian Open, but the coach put their parting of ways down to a lack of discipline.
The Spaniard expanded on the issues that lay at the heart of the breakdown in their relationship, pointing to what he perceived as a lack of professionalism from the German.
"We didn't finish in a bad manner... but we had different ways to understand the professional world, and the way to work every day," said Ferrero.
"He is German, with a Russian family, and it is just that we have different cultures. I have been working my whole life in a different way. I tried to transmit different things for him to be a bit more professional in different aspects... but it was just to improve.
"I wasn't criticising him. He didn't understand it like that, and we went our own ways."